Refining of stainless steels

ABSTRACT

In a process for refining stainless steel where oxygen and ammonia are introduced into a molten steel, e.g. for decarburisation and/or introducing nitrogen into the steel, the ammonia is introduced separately from the oxygen through a tuyere comprising at least two co-axial pipes. The ammonia is preferably introduced through a pipe which is not an outermost pipe. This procedure reduces the risk of the melt solidifying around the ammonia pipe and avoids the need to use combustible oxygen/ammonia mixtures.

United States Patent [1 1 [111 3,898,079

Eriksson Aug. 5, 1975 [5 REFINING 0F STAINLESS STEELS 3,706,549 12/1972Knuppel 75/60 [75] inventor: Lars Anders Eriksson, Hagfors,

Sweden Prmzary E.\'anz1net'-L. Dewayne Rutledge Assistant Examiner-PeterD. Rosenberg Asslgneei Uddeholms Aktlebolag, Hagfors, Attorney, Agent,or FirmCushman, Darby &

Sweden Cushman [22] Filed: Oct. 5, 1973 [30] Foreign ApplicationPriority Data Oct. 6, 1972 Sweden 12908/70 [52] U.S. Cl 75/60; 75/59[51] Int. Cl. C2lc 5/32 [58] Field of Search 75/59, 60

[56] References Cited UNITED STATES PATENTS 3,330,645 7/1967 De Moustier75/60 [57] ABSTRACT In a process for refining stainless steel whereoxygen and ammonia are introduced into a molten steel, e.g. fordecarburisation and/or introducing nitrogen into the steel, the ammoniais introduced separately from the oxygen through a tuyere comprising atleast two co-axial pipes. The ammonia is preferably introduced through apipe which is not an outermost pipe. This procedure reduces the risk ofthe melt solidifying around the ammonia pipe and avoids the need to usecombustible oxygen/ammonia mixtures.

4 Claims, 2 Drawing Figures This invention relates to a process forrefining stainless steels by simultaneous introduction of oxygen andammonia into the molten steel. i i 7 I Copending Application Ser. No.295,355 filed Oct. 5, 1972 by M. K. A. .Iohnsson and=L. A. Erikssondescribes a method 'by' which a melt of steel with a high percentage ofchromium, particularly a stainless steel,

is-decarburisedusing oxygen: and either ammonia or steam. Ammonia'isintrod-uced'for two reasons. Firstly, to lower the-partial pressure .of,carbon monoxide, because it develops hydrogen, which allows a highdegree of decarburisation, and, secondly to-increase the nitrogencontent of the steel. In principle, several methods are available forintroducing the oxygen and ammonia into the melt. For instance, asdescribed in Application Ser. No. 295,355, it is possible to introduceoxygen through a central pipe and the ammonia through a pipe whichenvelopes the oxygen pipe.. This arrangement is not entirelysatisfactory because the introduction of ammonia effects a high degreeof cooling, which may cause the opening between the oxygen pipe andammonia pipe to be blocked by the metal solidifying. It is alsopossible, in principle, to use a single pipe through which a gas mixturecontaining both oxygen and ammonia could be introduced. However, suchgas mixture is combustible, and it could burn in the delivery pipeeventually causing the pipe to be burnt out.

An object of the invention is to introduce the ammonia in a way whichwill eliminate the drawbacks mentioned above. The present inventionprovides a process for refining a steel containing chromium whichcomprises simultaneously introducing molecular oxygen and ammonia intothe steel in a molten condition, the oxygen and ammonia being introducedseparately from one another through different sections of a tuyerecomprising at least two coaxial pipes and presenting an innermostsection and an outermost section.

Specific embodiments falling within the framework of the invention willnow be described with reference to FIGS. 1 and 2 of the accompanyingdrawing, in which FIG. 1 is a sectional view through a tuyere made up oftwo concentric pipes, and

FIG. 2 is a sectional view through a tuyere with three concentric pipes.

In the tuyere of FIG. 1, a central tube 1 is enclosed by a coaxial tube2 of large diameter thus providing an outermost section 3 betweencentral tube 1 and coaxial tube 2 and an innermost section inside tube1.

In the tuyere of FIG. 2, a central tube 10 is enclosed by anintermediate coaxial tube 11 of larger diameter than tube 10 and by anouter coaxial tube 12 of larger diameter than tube 11. The tuyere thushas an innermost section inside tube 10, an intermediate section 13between tubes 11 and 10 and an outermost section 14 between tubes 12 and11.

The tuyere or tuyeres as illustrated in FIGS. 1 or 2 can be fitted in aconverter as described in Application Ser. No. 295,355, particularlywith reference to FIGS. 2 4 of the drawings of that application. Thenature of the chromium containing alloys to be treated and generalmethod of decarburisation which is normally preceded by a preheating inwhich any silicon present and, if necessary, some of the chromium, isoxidised, and normally followed by the steps of introducing nitrogen if.desired, reducing oxidised chromiumandgthen flush; 'irig outflresidualhydrogen is. described ,in detail in Ap- 'p lica tion SerQNo.295,355,;the disclosure of which is incorporated ,infthis;specificationby these references he'following Examples'are given to illustratetheinqn ExAMPLEi A stainless steeljcontaining about 1% by weig-ht carbon,0.7% by weight silicon, about l% by weight m anganese, about 18% byweight chromium, about 6% by weight nickel and the remainder iron andimpurities was rnelted inanelectricare furnace and placed in a converterof the type described'in FIG. 2 of. Application Ser. No. 295,355 butwith-tuyeres as illustrated in FIG. 1 of the accompanyingidrawing.

In thejnitialstage of refining the purpose of which is to raise thetemperature in the melt by burning off silicon, chromium and/or anyother readily oxidisable substance, undiluted oxygen is introducedthrough the central pipe 1 while a mixture of steam and oxygen isintroduced throught the outer section 3, the steam content beingsufficiently high that sufficient cooling is provided for the protectionof the tuyere and the surrounding lining, but not so high as to riskblocking by solidification of the melt. In the next stage of refining(decarburisation), steam and oxygen are introduced through the centralpipe 1 as well as through the outer section 3. In the next stage ofrefining (absorption of nitrogen), ammonia is introduced through one ofthe pipes, preferably the inner pipe 1. The quantity of ammoniaintroduced will depend on the desired percentage of nitrogen in thefinished steel. The ammonia in inner pipe 1 is preferably mixed withsteam. Through the outer section 3 a mixture of oxygen and steam isintroduced simultaneously and the mixing ratio is adjusted so as toprotect the tuyere and the adjacent parts of the converter fromoverheating as well as from blocking by solidification of the melt. Inthe next stage of refining (reduction), in which any oxidised chromiumis reduced and returned to the melt, ammonia possibly mixed with argoncan also be introduced, for example through outer section 3.

Ammonia can also be introduced simultaneously during this stage throughthe inner tube 1. In the final stage of refining (removing residualhydrogen), undiluted argon, or another gas without hydrogen is blownthrough inner tube 1 and outer section 3, to lower the percentage ofhydrogen in the melt.

EXAMPLE 2 The stainless steel and converter used are as described inExample 1 but the converter is fitted with tuyeres as illustrated inFIG. 2 of the accompanying drawing. The refining sequence is the same asthat described in Example I.

In the initial stage of refining, a mixture of oxygen and steam isintroduced through inner tube 10, undiluted oxygen throught theintermediate section 13 and an oxygen-steam mixture through the outersection 14.

pipe 10 whose dimensions are chosen so that the flow of ammonia will notbe excessive. Simultaneously, a mixture of oxygen and steam areintroduced through both intermediate section 13 and outer section 14.During the reduction stage, which comes next, undiluted ammonia isintroduced through inner tube 10. Simultaneously steam is introducedthrough intermediate section 13 and a mixture of ammonia and argonthrough outer section 14. An alternative in this stage is to introduceundiluted steam through inner pipe and intermediate section 13 and atthe same time undiluted enting an innermost section and an outermostsection, the ammonia being introduced through a section of tuyere otherthan the outermost section and the oxygen being introduced through theoutermost section.

2. A process according to claim 1 wherein the tuyere comprises threecoaxial pipes and presents an innermost section, an intermediate sectionand an outermost section and ammonia is introduced through at least oneof the innermost and intermediate sections and oxygen is introduced atleast through the outermost section.

3. A process according to claim 1 wherein a mixture of ammonia and steamis introduced through a section other than the outermost section.

4. A process according to claim 2 wherein a mixture of ammonia and steamis introduced through at least one of the innermost and intermediatesections.

1. A PROCESS FOR REFINING A STEEL CONTAINING CHROMIUM WHICH COMPRISESSIMULTANEOUSLY INTRODUCING MOLECULAR OXYGEN AND AMMONIA INTO THE STEELIN A MOLTEN CONDITION, THE OXYGEN AND AMMONIA BEING INTRODUCEDSEPARATELY FROM ONE ANOTHER THROUGH DIFFERENT SECTIONS OF A TUYERECOMPRISING AT LEAST TWO COAXIAL PIPES AND PRESENTING AN INNERMOSTSECTION AND AN OUTERMOST SECTION, THE AMMONIA BEING INTRODUCED THROUGH ASECTION OF TUYERE OTHER THAN THE OUTERMOST SECTION AND THE OXYGEN BEINGINTRODUCED THROUGH THE OUTERMOST SECTION.
 2. A process according toclaim 1 wherein the tuyere comprises three coaxial pipes and presents aninnermost section, an intermediate section and an outermost section andammonia is introduced through at least one of the innermost andintermediate sections and oxygen is introduced at least through theoutermost section.
 3. A process according to claim 1 wherein a mixtureof ammonia and steam is introduced through a section other than theoutermost section.
 4. A process according to claim 2 wherein a mixtureof ammonia and steam is introduced through at least one of the innermostand intermediate sections.